A construction machine including booms and a bucket such as a wheel loader, a shovel loader or the like has been used in a variety of utilization fields such as construction working site or the like as a working machine from the viewpoint of such advantages that it is constructed in a compact structure, it can turn with a small radius and it can be purchased at an inexpensive cost.
As shown in FIG. 11, this kind of construction machine is so constructed that booms 2 are turned upwardly and downwardly by actuating a boom cylinder 1 (raising of the booms is hereinafter referred to as `lifting`) and a bucket 4 is tilted (turned to a vehicle body side) and caused to dump scooped gravel or the like (reverse operation to tilting) by actuating a bucket cylinder 3. Thus, excavating operation (scooping operation) and loading operation are perfomed for gravel or the like by turning operations of the booms 2 and the bucket 4.
In general, when a scooping operation is performed for gravel, `lifting` of the booms 2 and `tilting` of the bucket 4 are alternately repeated. To this end, two methods as noted below are employed as a method of carrying out both the lifting operation and the tilting operation.
(1) An operator alternately actuates a boom operation lever and a bucket operation lever.
(2) In a case where a vehicle is equipped with a boom kick-out device for immovably holding the boom operation lever at a predetermined boom kick-out position, the booms are automatically lifted at a predetermined speed by operating the boom kick-out device. For the lifting operation, operator actuates only the bucket operation lever. Specifically, a bucket preference hydraulic circuit is used for the purpose of driving the working machine so that tilting operation and lifting operation are alternatively repeated by alternately repeating tilting of the bucket operation lever and releasing of the same (representative of shifting of the bucket operation lever to a neutral position).
With respect to the above-mentioned two methods, the second method identified by (2) can be easily practiced compared with the first method identified by (1), because a single operation lever is required therefor. When the second method identified by (2) is employed, a boom kick-out position is usually set to the maximum displacement position, causing an amount of pressurized hydraulic oil fed to the boom cylinder 1 to be maximized during a period of boom lifting as shown in FIG. 12 (periods represented by III and V in FIG. 12). Accordingly, during a period of boom lifting operation as mentioned above, a lift speed of the booms 2 becomes excessively high (to the highest speed) and this makes it very difficult for operator to tilt the bucket in harmonization with the lift speed during a subsequent period of bucket operation.
FIG. 11 illustrates a track B scribed by the blade edge of a bucket in accordance with the conventional method when a scooping operation is performed. In the drawing, reference character W designates an upper surface of gravel and reference character A does a line representing an ideal track. As will be apparent from the drawing, when the conventional method is employed, a tilt speed of the bucket does not follow a lift speed of the booms and therefore the track B scribed by the blade edge of the bucket is not only parted away from the ideal track A but also a period of dumping operation as represented by reference character V is required. Namely, with the conventional method, the lift speed is not harmonized with the tilt speed, resulting in a degree of fullness of the bucket becoming insufficient in the course of scooping operation. In the event of this insufficient fullness, operator actuates the associated lever to turn the bucket to the reverse side to tilting operation so that a shortage in fullness of the bucket is compensated. During a period of dumping operation that may be called useless period, a bucket vertical load F.sub.V is reduced as shown in FIG. 13, causing a slippage to be induced with fore wheel tires during the period V. Consequently, excavating operation can not be performed at a high operational efficiency.
The present invention has been made with the foregoing background in mind and its object resides in providing an apparatus for controlling a speed of a working machine in the form of a construction machine which assures that a bucket moves along an ideal track by allowing a lift speed of the booms to be harmonized with a tilt speed of the bucket and an occurrence of tire slippage requiring useless dumping is prevented.